150 



PRACTICAL COURSE IN BOTANY 



The different forms are characterized by 

 the angular distance between the points 

 of leaf insertion around the stem. In the 

 elm, basswood, and most grasses, they are 

 distributed in two rows or ranks on op- 

 ^^ posite sides of the stem, each just half 

 way round the circumference from the 

 one next in succession (Fig. 189), the 

 third in vertical order standing directly 

 over the first. In most of our common 



in making two turns round the stem, 

 the sixth leaf in vertical order stand- 

 ing over the first. This is called the five-ranked arrange- 

 ment, and is the most 

 common order among 

 dicotyls. 



169. Relation be- 

 tween the shape and 

 arrangement of leaves. 

 Phyllotaxy is of im- 

 portance chiefly on ac- 

 count of its influence 

 on the light relation of 

 leaves. A compact, 

 close-ranked arrange- 

 ment tends to shut off 

 the light from the lower 

 nodes, and hence, in 

 plants where it pre- 

 vails, the leaves are apt 

 to be long and narrow 

 in proportion to the 



frequency Of the Ver- FlG> 189 ._ Twig of a hackberry (Cdtiscinerea), 



tical rOWS. The yUCCa showing the two-ranked arrangement. Notice how 



~ , ,. the position of the stems and branches of the main 



Oleander, Canada flea- axis corresponds to that of the leaves. 



